Delafossite CuCoO2/ZnO derived from ZIF-8 heterojunctions as efficient photoelectrodes for dye-sensitized solar cells

To achieve high-performance dye-sensitized solar cells (DSSCs), it is essential to establish new and effective photoelectrode materials. Herein, we report the successful synthesis of heterojunctions including Cu-based delafossite oxide CuCoO2 and ZnO derived from zeolitic imidazolate framework-8 (ZIF-8). The layered polyhedral nanocrystals of CuCoO2 produced through a feasible low temperature hydrothermal process and the faceted nanocrystals of ZnO were achieved by heat treatment of ZIF-8. The composite heterostructures were applied as photoelectrodes in DSSCs assembled using dye N719 and a Pt counter electrode. The physicochemical characteristics (XRD, FESEM, EDAX, mapping, BET, DRS), dye loading, and photovoltaic properties (J–V, EIS, IPCE) of the fabricated materials were studied and fully discussed. Results revealed that addition of CuCoO2 to ZnO significantly improved the Voc, Jsc, PCE, FF, and IPCE. Among all cells, CuCoO2/ZnO (0.1 : 1) showed the best performance (PCE = 6.27%, Jsc = 14.56 mA cm−2, Voc = 687.84 mV, FF = 62.67%, IPCE = 45.22%) and acted as a promising photoanode in DSSCs.


Introduction
All living things on Earth receive energy from the Sun via the process of photosynthesis, making it the most essential and sustainable energy source currently available. 1 It is possible to directly convert the Sun's energy into useable heat or electricity. 2 Therefore, scientists and engineers are attempting to create solar-powered devices that can generate electricity more efficiently. 3 Solar radiation can be directly converted into electrical energy by a photovoltaic cell. Monocrystalline silicon solar cells are the 1st generation of solar cells. 4 These solar cells are more efficient than conventional solar cells as they are constructed from silicon wafers. 5 The high cost of their fabrication, however, increases the price of crystalline solar cells that are readily available on the market. 6 The second generation of solar cells are thin lm solar cells. The thin lm solar materials are in powder form, which give the cell additional exibility and lightness. 7 The low efficiency is the key challenge facing thin lm solar cells. 8 The DSSC belongs to the third solar cell generation. These solar cells have a higher efficiency than thin lm devices but a lower efficiency than crystalline solar cells. 9 O'Regan and Gratzel created the rst DSSC in 1991. 10 The DSSC can typically create electrons by the absorption of light, transport the electrons upon application of high voltages, and then return the electrons to the device at low voltages. The photoanode working electrode, which is a mesoporous semiconductor lm placed onto transparent, conductive glass (oen uorine doped thin oxide, FTO), the iodide/triiodide (I − /I 3 − ) redox electrolyte, and Pt counter electrode (cathode) are typically used to produce DSSCs. 11 One of the most important components of solar cells, the photoanode, is responsible for the charge collection and transfer. 12 The best DSSCs combine a photoanode with a high surface area (almost 1200 times that of a at electrode) with a dye of high extinction to provide good light gathering efficiency. 13 The light absorption can be extended into the red wavelengths indicating this combination enables adequate absorption over most of the visible spectrum. 14 The photons absorbed by dye molecules produce excitons that are split on a time scale of ten femtoseconds in the most effective cells, leading to charge separation efficiencies that are close to unity. 15 The difference between the rate of charge transport and the rate of charge recombination determines that how effectively the nal step (charge collecting) operates. 16 Iodide/triiodide (I 3 bonds. It has a direct and wide band gap (E g = 3.03 eV), extraordinary chemical stability, high exciton binding energy (60 meV), high optical gain, photo-luminescence, and piezoelectric properties, all of which make it very useful for a variety of optical devices. It is also non-toxic and abundant in nature. The inherent defects, such as O vacancies and Zn interstitials, which are present within the non-stoichiometric undoped ZnO thin lms, in general display n-type electrical conductivity with very high electron densities of about 10 21 cm −3 . 19 The ZnO lms are highly useful in solar cells due to their combination of high electrical conductivity, good visible transmittance, and effective light scattering properties. 20 Additionally, their pyramidal design was appropriate to utilize them as front electrodes (photoanode) and antireective coatings in solar cells. 21 Highly ordered and multidimensional ZnO nanostructures with hierarchical structures have been used frequently to increase the device efficiency in gas sensors, photocatalytic applications, and DSSCs. 22 One of the best alternatives to the TiO 2 semiconductor in DSSC manufacturing is ZnO, which has exceptional bulk electron mobility that is generally one order of magnitude better than the anatase TiO 2 . 23 Additionally, the ZnO lms have low electron trapped surface state densities, which improves cell performance. 24 However, ZnO only responds to ultraviolet (UV) light, which contributes for less than 5% of the solar spectrum's energy, due to its large E g = 3.03 eV. 25 According to a paper, adding different types of carbon or nitrogen to ZnO lattices can reduce the band-gap energy, extending the absorption edge into the visible light range and allowing for the use of 42% of solar energy. Either in situ synthesis or post-treatment can add carbon or nitrogen to ZnO. However, post-treatment oen results in random distribution throughout the prole from the surface to the center of a particle, whereas in situ doping typically necessitates a key synthesis environment.
Metal-organic frameworks (MOFs) are solid, porous nanomaterials made of inorganic hybrid centers or metal ions connected by organic linker molecules. In addition to their direct applications, MOFs may be easily and controllably employed as sacricial templates or precursors to generate a variety of hybrid inorganic nanomaterials. 26 ZIF-8, a Zn-containing N-rich MOF, is composed of Zn atoms attached to MeIm, with the formula Zn(MeIm) 2 . Without the need to a stabilizing agent or activation procedures like heating, microwave irradiation, or ultrasound, it is feasible to efficiently synthesize ZIF-8. 27 Recently, ZIF-8 was thermally treated to create porous ZnO photocatalysts, which improved methylene blue degradation. 28 Such a method would be investigated further for hybrid synthesis and heteroatom doping. 29 Compared to previous in situ or post-treatment techniques of carbon-and/or nitrogen-doping, the synthesis would be simpler. Taikei Enomoto et al. fabricated ZnO derived from ZIF-8 and employed it in DSSC photoanode and studied its performance. 30 Huifen Fu et al. used this method to achieve Ndoped ZnO and examined its capability in ethanol-sensing. 31 Scheme 1 illustrates the schematic ZnO production through heat treatment of ZIF-8. On the le side, the structure of ZIF-8 can be seen, in the middle part, the structure of ZnO obtained through heat treatment at 600°C is displayed, and in the right side, the structure of ZnO is presented from the top view.
Researchers used a variety of approaches, including introduction of dopants, modifying size and shape of semiconductor materials, and formation of heterojunctions, to promote absorption of these semiconductors in the visible light region and to suppress the charge carrier recombination. [32][33][34] Among them, creating heterojunctions between the photocatalyst materials and other semiconductor oxides would allow for increased activity by efficiently separating the charge-carriers through the creation of an interfacial electric eld. 35,36 Among numerous heterojunctions, p-n heterojunctions have demonstrated to be favorable for effective charge separation, extended lifetimes, and quick charge transfer. 37 Due to their broad usage in optics, magnetism, catalysis, and super-capacitors, copper oxide-based p-type delafossite materials (CuMO 2 , where M is 3d metal) have recently attracted increasing attention as electrodes. 38 In the past ten years, they have also achieved notable advancements in the conversion of solar energy, environmental protection, degradation of organic pollutants, and p-type DSSCs. 39,40 The CuCoO 2 is an example of copper oxide-based delafossites which has drawn particular interest due its efficiency, chemical and electrochemical stability, high catalytic activity, and superior corrosion resistance. It has been employed in a variety of applications such as photocatalysis showing good optical and photovoltaic properties. 41 Also, hole-doped polycrystals and thermoelectric materials were used in supercapacitors. 42 Additionally, it was utilized as a co-catalyst to increase the efficiency of charge separation and transfer in other materials, such as BiVO 4 lm for the oxygen evolution reaction. 43 Numerous techniques including solid state reaction, sol-gel, spray pyrolysis, magnetron sputtering, hydrothermal, and more recently co-precipitation at low temperature, have been described for the synthesis of delafossites. 44 The rhombohedral crystal structure of CuCoO 2 is depicted in Scheme 2. The monovalent cation Cu + is linearly coordinated with oxide O 2− anions in the delafossite layers, whereas the trivalent cation Co 3+ is octahedrally surrounded by six O 2− anions. Additionally, the O 2− bonds with three equivalent Co 3+ cations and one equivalent Cu + cation creates a mixture of deformed edge-corner sharing OCuCo 3 trigonal pyramids. 45 CuCoO 2 delafossite in particular can function as a potential ptype semiconductor for increasing the activity of DSSC due to its absorption in the visible region with a small bandgap energy of about 1.60 eV. However, because of the greater rate of electron-hole recombination, it has poor transport characteristics. 38 To overcome these difficulties, we present the development of heterostructures between ZnO as n-type wide bandgap semiconductor and CuCoO 2 as p-type narrow bandgap semiconductor for increasing the open-circuit voltage (V oc ) and enhancing photoelectric conversion efficiency (PCE) of DSSCs. 46 In this work, ZnO nanoparticles were successfully synthesized through a thermal treatment process of ZIF-8 as a self-sacricial template. N atoms were doped into the lattice of ZnO nanoparticles using this procedure. The copper oxide-based delafossite CuCoO 2 nanoparticles were also synthesized through a simple, low temperature hydrothermal method. Through combination of the mentioned materials, the CuCoO 2 / ZnO composites were produced with different mole ratios of CuCoO 2 to ZnO (0.05, 0.1, 0.15, 0.2) : 1. The composites were studied in DSSCs as photoelectrodes. The experimental results showed that CuCoO 2 /ZnO composite could serve in a promising way to enhance DSSC performance. The fabrication of CuCoO 2 / ZnO composite photoanode and its corresponding band diagram formed by CuCoO 2 (ref. 45) and ZnO 47 is depicted in Scheme 3. As it is illustrated in this diagram, the electrons excited to the N719 dye's conduction band are then moved to the CuCoO 2 /ZnO composite's conduction band, where they eventually nd their way to the FTO and external circuit. The higher level of the conduction band of CuCoO 2 compound aer forming heterostructure with ZnO prevents the return of electrons from the conduction band of ZnO to CuCoO 2 . Scheme 3 also depicts the CuCoO 2 /ZnO as a single layered photoanode that completely contacts the redox electrolyte, demonstrating how recombination is inhibited. 48

ZIF-8 synthesis
4 mmol of Zn(CH 3 COO) 2 $2H 2 O and 6 mmol of 2-MeIm were weighed and dissolved separately in 30 mL of methanol. Aer 12 hours under stirring at room temperature, the solution was transferred to the autoclave and heated in the oven at 100°C for 12 hours. Next, the pale-yellow crystals of ZIF-8 were ltered, washed three times with methanol and dried to obtain the solid powder ( Fig. 1).

ZnO synthesis
The synthesized ZIF-8 powder was poured into a crucible and heated at 600°C for 4 hours under air ow with heating rate of 2.5°C min −1 . Aer cooling down to room temperature, the Scheme 3 The fabrication of the dye-sensitized solar cell. On the right side, a DSSC can be seen, in which the photoanode is composed of the CuCoO 2 /ZnO composite shown in the form of pale-yellow circles along with N719 dye molecules (red circles). In the middle part, the FESEM micrograph of the composite photoanode is observed. On the left side, the band diagram has been exhibited for the heterostructure containing CuCoO 2 and ZnO (band gaps are taken from Tauc plots); including the electron path through the photoanode and the inhibition of recombination. Fig. 1 Schematic illustration of synthesis of the hexagonal wurtzite ZnO nanocrystals derived from ZIF-8 through heat treatment at 600°C. crystals were slightly grounded in an agate mortar. The obtained product was white colored powder of ZnO ( Fig. 1).

CuCoO 2 synthesis
5 mmol of Co(NO 3 ) 2 $6H 2 O and 5 mmol of Cu(NO 3 ) 2 $3H 2 O were respectively dissolved in 70 mL of water. Then, a certain amount of NaOH (1.46 g) was weighed and added to the solution. Aer half an hour, the solution was poured into an autoclave. It was heated in the oven for 6 hours at 160°C and then at 100°C for 18 hours. Subsequently, it was dried in a Petri dish for an hour and a dark green powder was obtained (Fig. 2).

CuCoO 2 /ZnO composites synthesis
The CuCoO 2 /ZnO composites were made by adding delafossite precursors to the solution containing ZnO crystals and then calcination at 600°C. The process was such that 8 mmol of Zn(CH 3 COO) 2 $2H 2 O and 12 mmol of 2-MeIm were separately dissolved in 50 mL methanol. Aer 12 hours under stirring at room temperature, the solution was transferred to the autoclave and heated in oven at 100°C for 12 hours. Next, delafossite precursors (Co(NO 3 ) 2 $6H 2 O and Cu(NO 3 ) 2 $3H 2 O) were added to a certain molar ratio of zinc to synthesize the CuCoO 2 /ZnO composites in (0.05, 0.1, 0.15, 0.2) : 1 ratios. The dried ZIF-8 crystals and these precursors were dispersed in 60 mL methanol and stirred for 2 hours. Subsequently, the solution was dried in oven at 80°C for 1.5 h and the obtained powder was moved into a crucible. The crucible was heated under air ow for 200 minutes at a temperature of 500°C with a heating rate of approximately 2.5°C min −1 . The temperature was then raised to 600°C at a heating rate of 5°C min −1 , held there for 20 minutes, and then cooled to 25°C in the open air. The resulting materials' color changed from dark green to light green with increasing zinc oxide content (Fig. 3).

DSSCs assembly
As it is illustrated in Fig. 4 (Fig. 4a). TiO 2 , ethyl cellulose, and SiO 2 were blended to achieve a viscous, transparent solution to create TiO 2 scattering paste. The TiO 2 lm was sensitized until maximal dye adsorption occurred when the photoanode was soaked in N719 dye solution (0.3 mM) for 20 hours (Fig. 4b). The dye-sensitized lms were then cleaned with ethanol to wash extra dye molecules. FTO glass was coated with a 0.5 mM H 2 PtCl 6 solution in 2-propanol to create the counter electrode. The counter electrode was then subjected to a 20 min treatment at 450°C. In the acetonitrilebased electrolyte (1 mL) of DSSCs, 4-tert-butyl pyridine (0.5 M), iodine (I 2 ) (0.05 M), and lithium iodide (LiI) (0.1 M) were present. At 120°C, the counter electrode and photoanode were connected, and the Surlyn spacer was used to complete the sandwich-type cell. To stop solution vaporization, electrolyte was injected into the internal area of each predrilled FTO before it was sealed.

Analysis methods
A eld emission scanning electron microscope (FE-SEM, TES-CAN MIRA-3) was used to analyze particle sizes and morphologies. Energy-dispersive X-ray spectroscopy (EDX, Oxford Instrument, and England) was employed to produce elemental analyses and mapping images. Results from X-ray diffraction (XRD) were used to identify each sample's crystallinity and crystalline phase using the INEL EQUINOX 3000 X-ray diffractometer at 30 kV and 30 mA with a Cu-K wavelength of 0.15406 nm in the 5-100°range. Each XRD pattern was examined using the X'Pert High Score program. UV-visible reectance characteristics are determined by the UV-visible diffuse reectance spectrum (UV-Vis DRS) (Avaspec-2048-TEC). The volume of pores, average pore diameters, and specic surface areas of samples were all calculated using volumetric adsorption analyzers (Belsorp mini and Finetec). Under high pressure, liquid nitrogen was used to calculate the total volume of pores. Electrochemical impedance spectra (EIS) and current densityvoltage (J-V) diagrams were analyzed under illumination by the electrochemical analyzer potentiostat galvanostat (OrigaLys). 100 mW cm −2 or AM 1.5 of simulated sunlight was produced by a solar simulator. With the use of an incident photon-to-current conversion efficiency (IPCE) spectrometer (PVE 300), IPCE spectra were aquired.

Structural analysis
The crystal structures of the pure ZnO, CuCoO 2 and CuCoO 2 / ZnO (0.1 : 1) were investigated by XRD analysis (Fig. 5).   52,53 where K = 0.89 is a constant, l represents Cu Ka = 0.15418 nm, q demonstrates the diffraction angle, and b displays the full-width halfmaximum (FWHM) (in radians), respectively. 54 The corresponding data are given in Table 1.

Morphological analysis
The morphology and microstructure of ZnO, CuCoO 2 , and CuCoO 2 /ZnO (0.1 : 1) composite samples were investigated through FESEM images. Fig. 6a shows that the ZnO sample consists of faceted crystals that are uniformly formed with high porosity, which can be due to its synthetic route from ZIF-8 heat treatment. 55 In Fig. 6b, layered polyhedral crystals of pure CuCoO 2 sample are observed. Fig. 6c displays the FESEM micrograph of composite CuCoO 2 /ZnO (0.1 : 1) which signies fabrication of uniform nanoheterojunction, where the zinc oxide and delafossite crystals are in close contact with each other on the surface. As it is depicted, we can see agglomerated particles in some places, but overall, we see high porosity in the composite.
For further investigation of the porosity of the composite sample, FESEM image was taken at another angle with a magnication of 2 mm (Fig. 6d). As we can see in the FESEM image, the porosity of the nal sample increases with the determined heating rate that takes place during the composite preparation process (the distances between the particles dene the porosity of the composite). Also, the yellow circles represent the delafossite but the red rectangles show a mixture of small spherical zinc oxide and delafossite, which form a high porosity structure for the composite (the increase in porosity was also determined by the BET test).
TEM images have been obtained to further explore the heterojunction structure formed between ZnO and CuCoO 2 delafossite (see Fig. 7a-c). As can be seen in Fig. 7a, the zinc oxide has a uniform structure, uniform particle dispersion, and suitable porosity. According to TEM image in Fig. 7b, delafossite nanoparticle indicates an agglomerated structure with polyhedron form. Aer adding the delafossite sample with a rigid and integrated structure and performing heat treatment, the nal composite has an evident porosity (Fig. 7c). Also, the presence of delafossite polyhedron can be seen next to porous zinc oxide particles, conrming they have formed a heterojunction with each other. Fig. 8a-c show the EDX spectra correlated to the elemental compositions for ZnO, and CuCoO 2 nanocrystals, and their nanocomposite CuCoO 2 /ZnO (0.1 : 1). Also, to investigate the element distribution in the samples, elemental mapping of a selected area is taken. It is evident that the ratio of elements is appeared as expected for ZnO nanoparticles, with the presence of a small amount of nitrogen remained in the structure aer heat treatment process of ZIF-8. Existence of zinc and oxygen is evidenced too. EDX spectrum of pure CuCoO 2 sample exhibits the presence of three elements (copper, cobalt and oxygen). The EDX of the composite CuCoO 2 /ZnO (0.1 : 1) conrms the presence of all of its constituent elements (Cu, Co, Zn, O, N).

Textural property
One of the most important characteristics of a nanomaterial for the use in electrochemical investigations is having a large surface area. The specic surface area and pore volume of synthesized ZnO, CuCoO 2 and CuCoO 2 /ZnO (0.1 : 1) samples were estimated using BET analysis. The N 2 adsorptiondesorption isotherms for ZnO, CuCoO 2 and CuCoO 2 /ZnO (0.1 : 1) nanocomposite along with the corresponding BJH (Barrett-Joyner-Halenda) pore size distributions are shown in Fig. 9. Adsorption-desorption graphs demonstrate that all of them belong to the typical type IV isotherm with H3 hysteresis on the basis of IUPAC classication. 56 The isotherms also demonstrate that the textures of produced samples are mesoporous. Mesoporous materials with high surface areas are anticipated to offer more accessible electrocatalytic active sites and effective channels for electron transfer during electrocatalytic activity in the DSSC photoanode. For samples ZnO, CuCoO 2 and CuCoO 2 /ZnO (0.1 : 1), the obtained BET specic surface areas are 43.87, 11.03, and 70.32 m 2 g −1 , respectively.
According to BJH, the pore diameter based on the sharpest adsorption peak for the samples are obtained to be 23.68, 18.57, and 24.02 Å for the ZnO, CuCoO 2 , and CuCoO 2 /ZnO (0.1 : 1), respectively. This result veries that addition of the CuCoO 2 nanoparticles to ZnO increases the pore diameter of the CuCoO 2 /ZnO (0.1 : 1) composite nanostructure (Fig. 9, insets). For samples ZnO, CuCoO 2 and CuCoO 2 /ZnO (0.1 : 1), the BJH pore volumes are 0.14, 0.0086, and 0.56 cm 3 g −1 , respectively, and the data are listed in Table 1. In general, it can be said that the composite fabrication boosts the surface area, pore diameter and pore volume. This leads to an increase in active sites along with an increase in dye loading, resulting in improved performance.

Optical properties
3.4.1. UV-Vis DRS spectra. The DRS spectra evaluate the signicance of the light scattering effect in enhancing the efficiency of DSSCs. 57 For strong light scattering, the sizes of the scattering particles must be large enough. Moreover, the structure's empty spaces result in many light reections that signicantly increase scattering and subsequently light absorption by the photoelectrode material. 58 As a result, adding the delafossite CuCoO 2 nanoparticles with good electronic properties (low band gap) to layered ZnO nanocrystals will improve the amount of light harvesting and light absorption at 400 nm in the composite. On the other hand, above 400 nm, the scattering inuence and also the inherent properties of nanoparticles lead to an increase in light absorption by the composite (Fig. 10, insets). The optical bandgaps of both materials are ascertained from the absorption spectra using the Mott and Davis formula: (ahn) n = (hn − E g ), where h is Planck's constant, a is the optical absorption coefficient, and n is the radiation frequency. The value of n depends on the type of transition that results in the absorption, and E g is the optical energy gap. 48 In our case, the value of n is 1/2 because CuCoO 2 and ZnO NPs are the indirect band gap semiconducting materials, in which the transition is carried out via an authorized direct energy level. 59 Tauc plot is (ahn) 1/2 as a function of photon energy hn, which is drawn in Fig. 10.
Optical energy gaps are measured through linear tting and extrapolation of the main absorption peak towards the x-axis for all samples. The E g values of 1.60, 3.03, and 2.51 eV are obtained for pure CuCoO 2 , ZnO, and CuCoO 2 /ZnO (0.1 : 1), respectively. These data are shown in Table 1, and the energy gap of ZnO is reduced as a result of CuCoO 2 addition into the ZnO lm. This is because formation of composite alters the sample's size distribution. The wavelength range of light absorption and the capacity of CuCoO 2 composites to absorb light are increased by its optimal band gap. Consequently, an optimal content of CuCoO 2 enhances the light absorption property. CuCoO 2 /ZnO (0.1 : 1) has the highest light absorption and the lowest optical band gap energy, implying it may have the highest photovoltaic efficiency.
3.4.2. Dye loading. The fundamental reason for the J sc change is increasing the amount of dye adsorption. Using UV-Vis spectra, following the desorption of dye molecules from the electrodes inside 0.1 M NaOH in water/ethanol solution with a molar ratio of 1 : 1, we evaluated the dye loading capabilities of each photoanode. Also, the UV-Vis peak intensity for the N719 dye at 515 nm is compared to assess the relative dye adsorption of each sample. Fig. 11a clearly demonstrates that how adding the ideal quantity of CuCoO 2 to the ZnO layer improves dye adsorption. The reason for increasing the amount of loaded dye as the CuCoO 2 dosage is raised may be explained by the fact that the density of the nanocomposite layer decreases as nanoparticles become looser with the addition of the CuCoO 2 nanoparticles while the bare ZnO layer exhibits a little larger density. This results in more dye molecules being adsorbed by semiconductor nanocomposites as there are greater gaps between the particles. Dye molecules may be uniformly distributed across the surface of the nanoparticle or may congregate in islands on particular crystallographic faces, leaving other faces vacant. It is clear that among all samples, the CuCoO 2 /ZnO (0.1 : 1) nanocomposite has the greatest dye loading. The amount of dye loading subsequently decreases as the amount of CuCoO 2 nanoparticles increases. This result is explained by improvements in total dye loading, light absorption efficiency, and dye aggregation. In the end, inhibiting dye aggregation on nanocomposites is more effective at preventing dye adsorption than aggregating nanoparticles.

Photovoltaic, EIS, and IPCE studies
Several pastes having different mole ratios of CuCoO 2 /ZnO ((0.05, 0.1, 0.15, 0.2) : 1) were made in order to investigate the impact of CuCoO 2 nanoparticles on the photovoltaic characteristics of the ZnO photoanodes in DSSCs. Energy efficiency was calculated according to: PCE= (J sc × V oc × FF)/P in , where P in is the total amount of light radiated to the cell (100 mW cm −2 , AM 1.5). 60 The current density-voltage (J-V) curve for each cell can be employed to calculate V oc and J sc . Fig. 11b displays the J-V curves of the DSSCs under illumination. The J sc , V oc , FF, and PCE data of the cells are summarized in Table 2. For the reference DSSC based on ZnO photoanode, the FF is determined to be 44.47% and the V oc = 597.00 mV, J sc = 11.51 mA cm −2 , PCE = 3.05%. Among all of the cells, the ZnO cell has the lowest photovoltaic values. V oc , J sc , and PCE rise with increasing percentage of the CuCoO 2 added to ZnO layer, achieving maximum values for the CuCoO 2 /ZnO (0.1 : 1) cell with V oc = 687.84 mV, J sc = 14.56 mA cm −2 , highest PCE of 6.27%, and FF = 62.67%. Nevertheless, there is a minor reduction in the photovoltaic characteristics when CuCoO 2 content reaches 0.2 (CuCoO 2 /ZnO (0.2 : 1)) with V oc = 641.37 mV, J sc = 11.81 mA cm −2 , PCE = 4.25%, and FF = 56.61%. It is important to remember that the reason for gradual increase of J sc in the CuCoO 2 /ZnO 2 (0.1 : 1) cell compared to ZnO cell is because the photoanode's specic surface area has been increased (proved by BET analysis). Moreover, UV-visible tests on sensitized ZnO and various CuCoO 2 /ZnO ((0.05, 0.1, 0.15, 0.2) : 1) photoanodes have demonstrated that the absorbance increases with increasing CuCoO 2 nanoparticle dosage up to CuCoO 2 /ZnO (0.1 : 1) content, demonstrating because of their greater specic surface areas, photoanodes with more nanoparticle contents loaded more dye molecules. Thus, a greater photocurrent is produced when more dye molecules are adsorbed.
EIS studies on the cells at varied CuCoO 2 /ZnO ((0.05, 0.1, 0.15, and 0.2) : 1) ratios have been carried out in order to better understand the impact of CuCoO 2 nanoparticle on ZnO-based DSSC operation in the presence of light at open-circuit voltage, between 1 and 10 5 Hz. Impedance spectroscopy is a powerful method to determine the charge transfer resistance at the semiconductor/electrolyte interface. 61 It has helped us to comprehend the characteristics of electron transport and the process of charge recombination. 62 Fig. 12a displays the Nyquist curves of the individual cells. Two semicircles with widths corresponding to the charge transfer resistance at the electrolyte/Pt-FTO interface (R 1 ) and the charge recombination resistance at the CuCoO 2 /ZnO ((0, 0.05, 0.1, 0.15, 0.2) : 1)/dye/ electrolyte interface (R 2 ) are shown in the high and medium frequency regions, respectively. The series resistance (R s ) of the cells is determined using the curve shi on the real impedance axis. R s is mostly ascribed to the cell's electrical contacts, substrate resistance, electrolyte resistivity, and wiring. 63 The parameters R s , R 1 , and R 2 for each cell are displayed in Table 2. It is found out that the CuCoO 2 /ZnO (0.1 : 1) photoanode, which has the most porous surface of all the cells and the lowest resistance, also has the shortest semicircle radius, indicating it is the best photoelectrode material. Table 2 indicates that the charge transfer resistance (R 1 ) and charge recombination resistance (R 2 ) decrease by adding CuCoO 2 into the ZnO matrix. Notably, it seems that the R 1 and  displays that the R 1 has been decreased by 58 and 52%, respectively, in the composite photoanodes. This result conrms that the R 1 value in the ZnO containing DSSC is very larger than those of the devices fabricated using CuCoO 2 /ZnO (0.1 : 1) and CuCoO 2 /ZnO (0.15 : 1) materials.
Among composite photoanodes, the lowest and the highest R 1 values are measured for the DSSCs composed of CuCoO 2 / ZnO (0.15 : 1) and CuCoO 2 /ZnO (0.2 : 1) photoanodes, respectively, conrming the smallest charge transfer resistance is measured for the DSSC based on CuCoO 2 /ZnO (0.15 : 1) photoelectrode (5.58 U cm 2 ). On the other hand, the C 2 charge capacitance value of this device (455.57 mF) is lower than that of the DSSC with CuCoO 2 /ZnO (0.15 : 1) photoanode (503.85 mF). As the R 1 and R 2 values of the devices assembled based on   CuCoO 2 /ZnO (0.10 : 1) and CuCoO 2 /ZnO (0.15 : 1) photoanodes are very close to each other, therefore, it can be found that the best performance of the champion DSSC with CuCoO 2 /ZnO (0.15 : 1) composite material is related to its low charge transfer resistance, suitably high charge recombination resistance, and the highest C 2 charge capacitance. IPCE spectroscopy was used to test the capacities of DSSCs to capture light. The IPCE spectra of ZnO and CuCoO 2 /ZnO ((0.1) : 1) based DSSCs are displayed in Fig. 12b. In the visible area, close to 520 nm, DSSCs exhibit a broad peak. Generally, improving the DSSC's light harvesting efficiency is another crucial element which inuences its performance. The greater J sc during short-circuit situation is indicated by the higher IPCE value. 64 The greatest IPCE absorption at 520 nm achieved by the DSSCs based on the CuCoO 2 /ZnO (0.1 : 1) and ZnO photoanodes, respectively, are 45.22% and 23.95%. The strong scattering impact of the highly ordered CuCoO 2 /ZnO composite structure and the ne dispersion of CuCoO 2 nanoparticles dispersed onto the ZnO particles with high specic area are responsible for this signicant improvement in the IPCE.

Conclusion
For highly effective DSSCs, new and efficient photoelectrode materials must be developed. In the current work, a novel, costeffective, feasible and effective DSSCs have been assembled using zinc oxide and copper based delafossite CuCoO 2 (CuCoO 2 /ZnO) composites. ZIF-8 was heated to create the ZnO nanoparticles, which were then combined with the CuCoO 2 material. In order to fabricate DSSC devices, CuCoO 2 /ZnO nanocomposite photoanodes, N719 dye, and Pt counter electrode were utilized. The FESEM images showed that the CuCoO 2 /ZnO nanocomposites formed uniform particles and the XRD data indicated the presence of polycrystalline ZnO nanoparticles. The existence of Cu, Co, Zn, O, and N was veried by EDX analysis, and UV-Vis DRS spectra revealed that CuCoO 2 / ZnO (0.1 : 1) absorbed the utmost light and had the lowest optical band gap (2.51 eV) energy. The surface area of the nanostructures increased as a result of the addition of CuCoO 2 nanoparticles to ZnO, so that the highest surface area of 70.32 m 2 g −1 was measured for the CuCoO 2 /ZnO (0.1 : 1). Moreover, the CuCoO 2 /ZnO (0.1 : 1) nanocomposite showed the highest dye loading among all the samples. According to photovoltaic studies, the DSSC with CuCoO 2 /ZnO (0.1 : 1) photoanode had the highest PCE of 6.27%, which was 2.05 times greater than that of the device based on the bare ZnO (3.05%). The CuCoO 2 / ZnO (0.1 : 1) composite's photovoltaic data were J sc = 14.56 mA cm −2 , V oc = 687.84 mV, and FF = 62.67%, respectively. The CuCoO 2 /ZnO (0.1 : 1) composite electrode, with the most porous surface among all the samples and the lowest resistance, exhibited the smallest semicircle radius, according to the EIS analysis. The high photoconversion efficiency has been described in terms of pore size, surface area, and IPCE = 45.22%. The ndings proved that CuCoO 2 /ZnO (0.1 : 1) composite performed the best among all other samples and therefore it would be a promising photoanode for DSSCs.

Data availability
The data corresponding to this paper can be provided on request.

Conflicts of interest
Authors declare that they do not have any conicts of interest.